Calculating a blogging streak -- pseudocode

streak.py

# A Monday preceding the first post on this blog. Used for calculating week numbers
# (Weeks start on Mondays)
FIRST_MONDAY = date(2000, 2, 14)

# Calculate week_number and streak_weeks for every post, in order. For calculating today's
# streak, we only need to know about the most recent post, but there's a certain amount of
# looking at other posts to figure that out.
# Note: this code looks at *all* posts because I want to calculate all streaks. For just
# today's streak, a quicker algorithm would run in reverse chronological order and stop at
# the first break.
current_streak = 0
latest_week_number = 0
most_recent_post = None
for post in posts_in_chronological_order:
    post_week_number = math.floor((post.date - FIRST_MONDAY).days / 7) + 1
    
    # Figure out the streak
    if post_week_number == latest_week_number:
        # Same streak as previous post
        pass
    elif post_week_number == latest_week_number + 1:
        # The streak is increasing!
        current_streak += 1
    else:
        # We must have missed posting for a week. Start a new streak
        current_streak = 1
    
    # Record streak and update tracking variables
    latest_week_number = post_week_number
    post.streak_weeks = current_streak
    post.week_number = post_week_number
    most_recent_post = post


# Calculate today's streak. Look at today's date and the most recent post
today_week_number = math.floor((TODAY - FIRST_MONDAY).days / 7) + 1

if today_week_number == most_recent_post.week_number:
    # If there's a post this week, the current streak is that post's streak
    streak_today = most_recent_post.streak_weeks
elif today_week_number == most_recent_post.week_number + 1:
    # If the most recent post is *last* week, and there hasn't yet been a post this
    # week, we can't be sure that the streak has ended (because there might be a
    # post before the week end). So the current streak is that post's streak
    streak_today = most_recent_post.streak_weeks
else:
    streak_today = 0

return streak_today